1. Field of the Invention
The present invention relates to a common mode filter, and more particularly relates to a structure of a thin-film common mode filter.
2. Description of Related Art
In mobile devices, deterioration in reception sensitivity to wireless signals is a problem and the deterioration is caused by the radiation noise generated internally from inside the devices. For suppressing the radiation noise, a common mode filter is mounted on a digital signal line (a differential transmission line) in the device. Recently, such a noise solution is commonly known.
For example, Japanese Patent Application Laid-open No. 2012-129665 describes a common mode filter that can attenuate common mode noise greatly. This common mode filter is constituted by connecting first and second common mode filter elements in series. The lengths of first and second connection conductors are set such that at a desired frequency, the phases of connection points between the first common mode filter element and the first and second connection conductors can be changed by an amount larger than 0 degrees and equal to or smaller than 90 degrees relative to the phases of connection points between the second common mode filter element and the first and second connection conductors. The second common mode filter element serves as a capacitive element (a capacitor) for the common mode and therefore equivalently serves as an LC filter, thereby attenuating common mode noise greatly.
In recent mobile devices, wireless communication techniques not only calling but also one-segment broadcasting, a GPS function, and Wi-Fi can be achieved by one device, and the devices internally generate radiation noise in various frequency bands. When these wireless communication techniques use different frequency bands, it is very important to determine in which of the frequency bands the radiation noise, generated internally from the device, needs to be suppressed.
The conventional common mode filter described above has a large attenuation amount and has a function of greatly suppressing noise at a specific frequency. However, there is a problem in that when noise solutions for plural frequency bands are required to improve the reception sensitivity to wireless communication, the conventional common mode filter cannot provide a sufficient effect of suppressing noise.